The requirement of PET film to shorten the plastic film is to be stable at room temperature, to shorten when heated (above the glass transition temperature), and to produce more than 50% thermal contraction in one direction is ideal. The characteristics of heat shrinkable plastic film packaging are: the body is transparent, showing the image of the product; the packaging is tightly packed, with good anti-scattering properties; rain-proof, moisture-proof and mildew-proof; non-recovery, it has a certain anti-counterfeiting function. Thermally shortened plastic films are often used in the packaging of convenient food, beverages, electronic appliances, metal products, etc., especially shortening labels is the most important application area.

In addition to being used for shortening labels, heat-shrinkable plastic films have also begun to be used in the outer packaging of daily commodities in recent years. Because it can not only protect the packaged items from impact, rain, moisture, and rust, but also make the goods win users with exquisitely printed outer packaging, and at the same time, it can perfectly show the good image of the manufacturer. At present, more and more packaging manufacturers choose printed shortened film to replace traditional transparent film. Because printing shortening the film can improve the appearance level of the product, it is conducive to the advertising of the product, and can make the trademark brand create a deep image in the minds of consumers.

Thermally shortened plastic films are usually made from amorphous plastics, such as polystyrene, polyvinyl chloride, and PVDC. Polystyrene (PS) shortening film has low strength and impact resistance, so it is rarely used; while polyvinyl chloride (PVC) is not conducive to recycling and does not meet the needs of environmental protection. In foreign countries, especially in Europe, polyvinyl chloride (PVC) plastic films have been prohibited from being used in packaging, especially food packaging.

Polyester (PET) heat shrinkable film is a new type of heat shrinkable packaging material. Polyester (PET) has become an ideal substitute for polyvinyl chloride (PVC) thermally shortened film in developed countries due to its easy recycling, non-toxic, odorless, and good mechanical properties, especially in line with environmental protection.

Copolymerization modification of thermally shortened polyester (PET) films

Polyester (PET) film is a crystalline material. Usually, polyester film can only obtain a thermal shrinkage rate of less than 30% after special technical treatment. In order to obtain polyester films with a higher thermal shrinkage rate, it is necessary to modify them. That is, in order to prepare a polyester film with a high thermal shortening rate, it is necessary to carry out copolymerization modification of polyethylene terephthalate, which is a common polyester. The maximum thermal shortening rate of the copolymerized PET film can be as high as 70% or more.

Usually polyester is usually obtained by esterification and polycondensation of terephthalic acid (PTA) and ethylene glycol (EG), and it is classified as a crystalline polymer (strictly speaking, it is a polymer in which crystalline region and amorphous region coexist) . The so-called copolymerization modification means that in addition to the two main components of terephthalic acid (PTA) and ethylene glycol (EG), the third or fourth component is introduced to participate in the copolymerization, with the intention of generating asymmetric molecules. layout to form an amorphous PET copolymer.

The introduced third or even fourth monomer can be a diacid or a diol. Among them, dicarboxylic acids include isophthalic acid, malonic acid, succinic acid, adipic acid, sebacic acid, etc.; dihydric alcohols include neopentyl glycol, propylene glycol, diethylene glycol, 1.4 cyclohexane Dimethanol etc. For example, when it is modified by copolymerization with dicarboxylic acid (Acidic), the obtained PET copolymer is called APET; It is called PETG.

Among the third monomers introduced above, the most commonly used dicarboxylic acid is isophthalic acid (IPA). The addition of IPA can change the tight structure of polyester symmetry, damage the regularity of the macromolecular chain, and then reduce the force between macromolecules, making the polyester molecular structure more contrasting and graceful. At the same time, due to the introduction of IPA, it is difficult for polyester to nucleate and crystallize, and with the addition of the amount of IPA introduced, the APET copolymer transitions from some crystalline to amorphous polymers, because the crystallization capacity of this modified polyester APET is reduced, no The set area becomes larger, so it can be used to make high shrinkage films. It is recommended that the participation amount of the IPA be about 20%.

The introduced third monomer can also be a diol. The most commonly used diol is 1.4 cyclohexanedimethanol (CHDM). In the process of polyester copolymerization, the addition of CHDM will have a great influence on changing the tg, tm and crystallization rate of polyester. With the addition of CHDM content, the melting point of the copolyester PETG decreases, the glass transition temperature increases, and the copolymer becomes an amorphous structure. However, the addition of 1.4 cyclohexanedimethanol (CHDM) must be controlled within an appropriate range, and it is usually recommended that the addition of CHDM be 30-40%. This glycol-modified PETG can not only be used to prepare high-shrinkage films, but also can be used to produce heat-sealing films, high-brightness films, etc., and has a wide range of uses.

Shortening mechanism of thermally shortened films

Taking the production technology of BOPET film as an example, the production process is to first dry the PET copolymer resin, then melt and extrude it in an extruder, cast through a die/cold drum, and then heat the cast to the glass transition temperature. Within a certain temperature range above and below the melting temperature, and under the action of external force, a certain multiple of unidirectional or bidirectional stretching is carried out, usually 3.5 to 4.0 times of lateral stretching is required. After stretching, the PET macromolecular chain is oriented in the direction of the external force, and then it is cooled and shaped, so that the oriented PET molecular structure is "frozen" and shaped. This high elastic deformation under the action of external force has a "recall effect" of thermal shortening. When the film with "recall effect" is reheated above the stretching temperature, the frozen macromolecular orientation structure begins to relax , Microscopically, it is manifested as a shortening of the PET film. It is worth mentioning that the thermal shrinkage of this PET film is mainly contributed by the oriented amorphous part. This is why the thermal shrinkage rate of copolymer-modified amorphous PET films (APET or PETG) is much greater than that of conventional crystalline PET films. Therefore, the purpose of greatly improving the thermal shrinkage rate of the film can be achieved by adding an oriented amorphous region in the film. As mentioned above, the thermal shortening rate of polyester film is usually only below 30%, while the thermal shortening rate of copolymer-modified polyester can be as high as 70% or more.

Production technology of thermally shortened polyester film

The production technology process of thermally shortened polyester film is as follows:

The production technical conditions of thermally shortened polyester film and its influence are briefly introduced as follows:

1. Drying treatment: Because the polyester macromolecular chain contains ester groups, it tends to be hygroscopic. In the case of heating, even if there is a small amount of moisture, it is easy to hydrolyze. As a result, during the molding process, a lot of bubbles will occur, affecting normal production; at the same time, due to hydrolysis and degradation, the molecular weight is reduced, and the PET product is suddenly inferior. Therefore, it is necessary to carry out drying treatment before melt extrusion processing. It is recommended to use vacuum drum drying, the drying temperature is 70~75℃, and the drying time is more than 6h.

2. Melt extrusion: the co-polymerized PET resin after drying treatment can join the single-screw extruder for melt extrusion. °C.

If a vented twin-screw extruder is used for melt extrusion, the vacuum drum drying system can be omitted. Since the vented twin-screw extruder is usually provided with two vents, they are respectively connected with two vacuuming systems. The water contained in the PET resin and the low-molecular substances generated during the melt extrusion process can be removed by vacuum pumping to achieve the same effect, and can greatly save investment and operating costs.

3. Casting cast sheet: The melt extruded through melt metering pump, filter and melt pipeline enters the coat hanger die and is cast from the die lip to the cooling drum to form a cast sheet. The cooling water temperature of the cooling drum is controlled at about 30°C.

4. Uniaxial stretching: High thermal shrinkage polyester films often require unidirectional shrinkage, especially transverse thermal shrinkage. The preheating temperature of transverse stretching is 90~100℃, the stretching temperature is 105~110℃, and the stretching ratio is 3~3.5 times. Chain clips must be air-cooled, and the temperature of the clips should be controlled below 110 °C to avoid the copolymer resin sticking to the clips. The stretched PET film is immediately cooled without heat setting.

5. Rewinding and slitting: The uniaxially stretched PET film is subjected to online thickness measurement, traction and winding, and finally slitting, inspection and packaging according to the standards required by users, which is a thermally shortened film product.

With the continuous development of China's plastic packaging industry and the continuous improvement of people's living standards, most beverages such as juice and soda are packaged in PET plastic bottles with excellent general performance, good isolation function, non-toxic and tasteless, and the amount is very considerable. According to statistics, the current national consumption of PET beverage bottles is at least 250,000 tons, and the amount of PET heat-shrinkable label film that is matched with it can reach 20,000 to 30,000 tons. In addition, since China's entry into the WTO, foreign trade has continued to expand. In order to improve the image and value of export commodities, it is also necessary to use higher-level packaging materials such as polyester heat shrinkable films.